1. Field of the Invention
This invention relates to a thermal recording apparatus which can produce images having precisely multiple tones (hereinafter, abbreviated as "multitone").
2. Description of the Prior Art
In recent years, thermal recording apparatuses have advanced with respect to full-color and high-speed recording, but high-precision multitone recordings have been in demand so as to obtain good half-tone recorded images.
FIG. 6 shows a thermal recording apparatus. The apparatus of FIG. 6 comprises a thermal head 1 having a plurality of heating element groups 1R which are arranged in a row, a platen 2, and a thermal transfer ink sheet 3 having a base film 3a and thermal transfer ink 3b applied thereon. The thermal transfer ink sheet 3 and a paper sheet 4 are inserted between the thermal head 1 and the platen 2. The platen 2 is urged against the thermal head 1 to ensure sufficient contacts between the paper sheet 4 and the thermal ink sheet 3 and also the thermal ink sheet 3 and the heating element groups 1R. FIG. 12 shows the appearance of the thermal head 1.
In the thermal head 1, as shown in FIG. 8, there are N number of the heating element groups 1R.sub.1 -1R.sub.N each consisting of M number of resistors or heating elements R.sub.1 -R.sub.M which are connected in parallel. The structure of the heating elements R.sub.1 -R.sub.M is illustrated in FIG. 7A. In each of the heating element groups 1R.sub.1 -1R.sub.N, the heating elements R.sub.1 -R.sub.M elongate along the direction of the broken lines shown in FIG. 7A, and are arranged in the direction perpendicular to the broken lines of FIG. 7A. In other words, the heating elements R.sub.1 -R.sub.M of all heating element groups 1R.sub.1 -1R.sub.N are arranged in a row. Each of the heating elements R.sub.1 -R.sub.M has the center portion 1e functioning as a heating portion, and two end portions 1f functioning as terminals. The heating portion 1e becomes wider nearer the terminals 1f, and are narrowest at the midpoint between the terminals 1f. This configuration results in a low resistance in the heating portion 1e and a high resistance at the midpoint between the terminals 1f, as shown in FIG. 7B (wherein the horizontal axis is the distance X from one of the end portions 1f along the arrow of FIG. 7A, and the vertical axis the resistance R). When a constant voltage is applied to the heating elements R.sub.1 -R.sub.M for a fixed time period, the amount of heat produced becomes higher as the resistance becomes higher, and therefore the density of the heat produced becomes higher near the midpoint of the heating elements R.sub.1 -R.sub.M. By utilizing this action and varying the time the voltage applied to the heating elements R.sub.1 -R.sub.M, the recorded area per dot or pixel can be freely changed according to the amount of generated heat. This is because the amount of heat generated according to the period of time the voltage is applied to the heating elements R.sub.1 -R.sub.M concentrates in the midpoint of the heating elements R.sub.1 -R.sub.M, thereby enabling the apparatus to perform multitone recording. The above is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 60-58,877, 1985.
The heating element groups 1R.sub.1 -1R.sub.N are disposed on one insulating substrate 1S (FIG. 12), and commonly connected at one of the terminals 1f to a printer power source (not shown), and the other terminals are respectively connected to output terminals of drive circuits 6 (FIG. 1). The input terminals of the drive circuits 6 are connected to output terminals of latch circuits 7. The input terminals of the latch circuits 7 are in turn coupled to output terminals of shift registers 8. The drive circuits 6 receive in parallel an enable signal supplied from a CPU (not shown) so as to be controlled independently from each other. To the latch circuits 7, a strobe signal is supplied from the CPU, and to the shift registers 8, a clock signal is supplied from a clock generator (not shown).
The input terminal of each of the shift registers 8 is connected to an output terminal of a comparator 11 of a signal circuit unit 5. The signal circuit unit 5 further comprises a RAM 9 and a 12-bit counter 10. The RAM 9 stores multitone data (multiple bits) of pixels of one line to be printed (i.e., multitone data for heating elements R.sub.1 -R.sub.M of all the groups 1R.sub.1 -1R.sub.N). As described in detail later, the output of the counter 10 is supplied to the RAM 9 as address data indicative of the address in the RAM 9 at which multitone data for one heating element is stored, and also to the comparator 11 as threshold data which is to be compared with multitone data supplied from the RAM 9. The comparator 11 outputs sequentially the result of the comparison to the shift registers 8 as print data signals.
The multitone data signals input to the respective shift registers 8 are sent as parallel signals to the corresponding latch circuits 7, the drive circuits 6 are driven by the multitone data signals sent from the latch circuits 7, and the heating elements R.sub.1 -R.sub.M are energized in accordance with the multitone data signals, whereby thermal recording is performed.
With reference to FIGS. 9, 10 and 11, the operation of the apparatus will be described. In order to simplify the description, it is assumed that the multitone data stored in the RAM 9 are 2-bit data, and that N is 4 and M is 256. That is, the apparatus has four heating element groups 1R.sub.1 -1R.sub.4 each of which consists of 256 heating elements R.sub.1 -R.sub.256, and each line consists of 1024 pixels (or heating elements).
As shown in FIG. 9, the multitone data corresponding to the one line of 1024 heating elements are stored sequentially from address "0" in the RAM 9. The 256 data stored from address "0" to address "255" correspond to the heating element group 1R.sub.1, and the next successive groups of 256 data correspond to the heating element groups 1R.sub.2, 1R.sub.3 and 1R.sub.4, respectively. The 5th to 12th bits of the output of the counter 10 are used for designating the 3rd to 10th bits of an address of the RAM 9 (in other words, for designating one of the heating elements R.sub.1 -R.sub.256 of one of the heating element groups 1R.sub.1 -1R.sub.4). The 1st and 2nd bits of the output of the counter 10 are used for designating the 1st and 2nd bits of an address of the RAM 9 (in other words, for designating one of the heating element groups 1R.sub.1 -1R.sub.4. The multitone data stored at the addresses which are designated by the combination of the 1st, 2nd and 5th to 12th bits of the outputs of the counter 10 are supplied sequentially to the comparator 11. The numbers in parentheses in FIG. 9 are decimal representations of the stored values. The 3rd and 4th bits (threshold data) of the output of the counter 10 are input to the comparator 11.
In the comparator 11, the multitone data corresponding to each of the heating elements R.sub.1 -R.sub.256 of one heating element group are sequentially compared with the threshold data, and the results are sent sequentially to the shift registers 8. This will be described more specifically. When the threshold data from the counter 10 to the comparator 11 is 0, the multitone data is compared with 0. If the multitone data is 0 or greater, the output of the comparator 11 is "1", and if it is less than 0, the output is "0". Each time 256 multitone data have been compared, the threshold data (3rd and 4th bits) from the counter 10 to the comparator 11 is advanced one by one up to 2 (i.e., from 0 to 1, and from 1 to 2), and the output of the comparator 11 varies as indicated in column C of FIG. 10.
The outputs of the comparator 11 are sequentially input to the shift registers 8, and the outputs of the registers 8 are supplied in parallel to the corresponding latch circuits 7, and then supplied to the driver circuits 6. An enable signal is sent in sequence to select one of the driver circuits 6. One of the driver circuits 6 which receives the enable signal is set to drive the first heating element group 1R.sub.1. The outputs of the selected driver circuit 6 have a waveshape as shown in column D of FIG. 10. Each of the heating elements R.sub.1 -R.sub.256 of the heating element group 1R.sub.1 is driven respectively by the outputs of the selected driver circuit 6 in which the pulse width corresponds to the multitone data stored at the corresponding address of the RAM 9, so that each of the heating elements R.sub.1 -R.sub.256 generates heat, the amount of which corresponds to the pulse width. This results in the recorded area per pixel varies in 4 levels, as indicated in column E of FIG. 10, according to the pulse width, whereby the tone of each pixel is controlled in 4 levels.
When the contents of the 1st and 2nd bits of the output of the counter 10 are advanced from 0 to 1, the above-described series of operations are performed against the second heating element group 1R.sub.2. In this way, one line is recorded by performing the above-described series of operations sequentially for all the heating element groups 1R.sub.1 -1R.sub.4, and by repeating recording lines while forwarding the paper sheet 4 in the direction of the arrow of FIG. 11, multiple lines are printed to form an image as shown in FIG. 11.
In the configuration described above, since the heating elements are divided into multiple groups which are driven separately, there is sufficient time for the heating elements to cool after heating. However, the heating elements R.sub.1 and R.sub.256 on either end of the heating element groups 1R.sub.1 -1R.sub.4, in each of which the heating elements are driven at the same time, receive thermal interference from adjacent heating elements on only one side. That is, since the radiation of heat generated in the end heating elements R.sub.1 and R.sub.256 to the outside of the group is large, the recorded area per pixel is less than that compared with those recorded by the heating elements R.sub.2 to R.sub.255. Even when all the heating elements R.sub.1 to R.sub.256 are driven by pulses of the same width, resulting in printing gaps G (white lines along the direction of printing (arrow in FIG. 11) on the recording surface of the paper sheet 4. The positions of the printing gaps G correspond to the boundaries between the heating element groups 1R.sub.1 -1R.sub.4. Thus, a conventional thermal printing apparatus has a drawback in that the printing quality is not of a sufficient high quality.
In order to overcome the above-mentioned drawback, an improved method is proposed in Japanese Laid-Open Patent Publication (Kokai) No. 61-224,772, 1986. In this method, heating elements at the both ends of a group of heating elements which are driven at the same time, are driven again immediately after all elements of the group have been driven. This method may be effective in correcting printing gaps in binary tone printing. In multitone printing, however, the period of driving a heating element depends on the recording tone (i.e., low tone: short, high tone: long). Consequently, it is difficult to obtain a good well-balanced correction at all tones in the proposed method.
Another technique is proposed in which heating elements at the ends of a heating element group are different in shape from other heating elements of the group (Japanese Laid-Open Patent Publication (Kokai) Nos. 61-144,367, 1986 and 61-185,462, 1986). In this technique, however, heat generated in each of the end heat elements is always corrected so that it is at a fixed ratio to that generated in the other heating elements, irrespective of tones. As in the above method, therefore, it is difficult to obtain good well-balanced correction at all tones. In order to obtain an optimum shape of end thermal elements for well-balanced printing gap correction in the binary tone printing, moreover, cut and try designs must be repeated.